Bracuiovopa. | LOWER PALAXOZOIC MOLLUSCA. 225 
quadrate pair of muscular impressions, reaching less than half the length of the shell, the length and width 
of the pair being nearly equal; the lateral boundaries are formed by the deep impressions of a sigmoidal ridge 
extending from the cardinal tooth on each side, each impression being slightly divided into two unequal lobes, 
anteriorly by the slight projection of an oblique narrow portion, diverging from the anterior lateral angles ; in 
fine specimens, a faint, transverse, arched furrow separates the small posterior adductor impressions, which are 
longitudinally wrinkled, from the smoother and larger anterior portion: interior of receiving valve with two short 
cardinal teeth diverging at 105°, from which extend two extremely long, narrow, elliptical, muscular impressions, 
extending from more than half, to nearly two-thirds, the length of the shell, each lobe marked with faint radiating 
sulci, the pair being separated by a flat, acutely angular space, a little wider than one of the impressions at the 
anterior end, and extending to the point of the beak; a narrow space of the margin finely plicated, the rest of 
90 
the surface nearly smooth, or very faintly radiated. Width ten lines, proportional length of receiving valve =, 
of entering valve =, depth =. 
The backward curving of many of the lateral strize to terminate on the hinge-line, is more remarkable in 
this species than any other I know of, and suggests the specific name. Something of the same sort may be 
seen in a less degree in the O. swhquadrata (Hall), and one of the shells figured in the Mem. Geol. Surv. 
(Vol. II. pt. 1. t. 20. fig. 9.) as the O. testudinaria, from both of which the great general depression, the total 
absence of mesial fold, the concavity of the entering valve, great length of the narrow diverging muscular 
impressions in the receiving valve, &c., easily distinguish the present species. The same characters and back- 
ward curving of the lateral strize separate it from the O. protensa. 
Position and Locality.—Occurs in extraordinary profusion in the Bala flags of S. end of Pen y Gaer, near 
Cerrig y Druidion, Holyhead Road, Denbighshire, closely covering extensive surfaces of the beds; Bala flags 
at Hafod Evan, Penmachno, Caernarvonshire ; Bala schists of Cefn-y-coed, Llangedwyn, Montgomeryshire ; 
Bala schists W. side of Garn Brys, S. W. of Cernioge; abundant in the Bala schists of Bwlch, Llandrillo, 
Corwen, N. Wales; Bala schists of Llanwddyn, EK. of Berwyn mountains; Bala schists of Das Eithin ridge, 
Hirnant, Montgomeryshire; Bala schists of Pen Cerrig Serth, Builth, Radnorshire; Bala schists of Milltir 
Cerrig, Llangynnog, Montgomeryshire; Bala schists of Bwlch y Groes, S. of Bala, Merionethshire; Bala 
rocks of Alt yr Anker, Meifod, Montgomeryshire; schists of Alt tre Ffynnon; Bala limestone of Pentre 
ewm dda, S. of Glyn Diffwys, N. Wales; Bala limestone of Llanfyllin, Montgomeryshire; common in the 
Bala quartzite of Carn Goran, Cornwall. 
ORTHIS REVERSA (Sa/t.) 
Ref.—Salter in Sil. Foss. Irel. t. 5. f. 2. 
Sp. Ch.—Transversely broad-oval, or slightly quadrate when old, nearly orbicular when young; hinge-line 
very short ; cardinal area low, triangular, three times wider than high, and inclined backwards at 130° to the 
plane of the margins ; opening narrow, triangular: receiving valve moderately convex towards the beak, which 
is small and pointed, with a wide flattened triangular depression, increasing in size towards the front margin ; 
sides slightly convex; entering valve much deeper than the receiving one; profile regularly arched; greatest 
depth about half way between the beak and front margin; obtusely subcarinated, rounded along the middle ; 
sides slightly convex, and sloping rapidly to the margins; margins nearly level at the sides, raised at front 
towards the entering valve in a small rounded wave; surface radiated with narrow, minutely granular ridges, 
separated by flattened spaces, about equalling them in width ; strize subequal in the rostral portion, but each pair 
receiving one, or sometimes two or three intermediate smaller ones, towards the margin, without distinct 
branching; fifteen or sixteen longitudinal strize in two lines, at three lines from the beak ; internal cast of 
receiving valve, with two very short divergent cardinal teeth, from which, on each side, a. low tumid boundary 
to the muscular impressions (forming a shallow sulcus in the cast) extends forwards and outwards, enclosing 
a longitudinally oblong space, nearly half the length of the shell; cast of entering valve with a very small 
central pit of a rostral tooth, from which a shallow sulcus of an obtuse mesial ridge extends rather less 
than one-third the length towards the margin; interior of both valves nearly smooth, except towards the 
[rasc. 11. | Ge 
